The Niger Delta Avengers, a militant group in Nigeria's oil-rich Niger Delta, has urged President MuhammaduBuhari to visit the country's south-east to witness the scale of damage caused by years of oil spills.

The NDA said in a statement that the president should visit areas where facilities owned by foreign oil corporations are located.

"These are just the export terminals, [we are] not talking about the numerous oil and gas flow stations and the oil Wells that criss-cross the length and breadth of the Niger Delta," said the group in a statement. "Mr President, come and see for yourself what the host communities are going through in the hands of Nigeria government and the multinationals."

The group also urged Buhari to call for a referendum on whether Nigerians still want to be part of the country. The NDA suggested that Nigeria followed the example of the UK, which, on 23 June, held a referendum for its citizens on whether they wanted to remain in the EU.

The request for a referendum was also put forward by members of a pro-Biafran movement, who are calling for a vote on a possible breakaway of Biafran territories forcibly annexed to modern-day Nigeria during British colonisation.

Buhari had been scheduled to travel to Ogoniland, in the Niger Delta, to discuss with regional leaders ways to tackle renewed violence in the region and to launch a clean-up operation in areas polluted by years of oil spills. The leader, however, cancelled the visit.

The militants last claimed responsibility for an attack on 15 June. Nigeria, Africa's biggest oil producer, has already been negatively affected by renewed violence in the region as oil production dropped to 1.4 million barrels per day, from 2.2m bpd. Buhari accused NDA of vandalism and warned that the Nigerian government would deal with the militants in the same way it is tackling Boko Haram in the country's north-east.

Militant groups in the Niger Delta took hold in the early 2000s following the deterioration of people's living conditions worsened by environmental problems and oil spillages blamed on foreign oil exploration companiesGetty

Militant groups in the oil-rich Niger Delta region took hold in the early 2000s following the deterioration of people's living conditions blamed on the increase of oil-related activities by foreign exploration corporations. Tensions flared up in the local communities as some ethnic groups felt they were being exploited and did not benefit from the crude oil on their land.

The repeated oil spills that considerably damaged the environment and affected people's health further deepened the communities' frustrations. After being elected in 2015, Buhari extended a 2009 amnesty granted to 30,000 former militants in the area.